Acid-resistant catalysts for the direct hydrogenation of fatty acids to fatty alcohols

ABSTRACT

Process for the production of a new copper (II) chromite spinel catalyst using colloidal silica gel and to its use for the direct fixed-bed hydrogenation of fatty acids to fatty alcohols of corresponding chain length.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a new acid-resistant copper chromite spinelcatalyst for the direct hydrogenation of fatty acids to fatty alcohols.

2. Statement of Related Art

Fatty alcohols, i.e. predominantly linear, monofunctional alcoholshaving chain lengths of 8 or more carbon atoms, and their production aredescribed in detail in the literature, for example, in UllmannsEncyklopadie der technischen Chemie, 4th Edition, Vol. II, pages 427 to445. Preferred starting materials for their production are the fattyacids and fatty acid mixtures occurring in natural fats and/or oilswhich may be converted into fatty alcohols of corresponding chain lengthby catalytic hydrogenation. Through the use of the fatty acids to bereduced in the form of their methyl esters, the catalysts in particularare protected against aggressive attack by the free carboxyl group, sothat industrial processes can be operated for sufficiently long periodswith satisfactory volume-time yields. Today, therefore, the predominantquantity of native fatty alcohols is produced from fatty acid methylesters by a process in which the distilled methyl esters are passed inthe liquid state, together with a large excess of hydrogen, over fixedlyarranged copper-containing mixed oxide catalysts, such as for examplecopper/zinc catalysts, at temperatures above 200° C. and under pressuresof from about 250 to 300 bar.

The copper-mixed oxide catalysts obtained by coprecipitation via the wetroute are used as particulate catalysts or extrudates and, before use,are generally reduced in the plant or installation. They are notacid-resistant. Accordingly, it has not hitherto been possible inpractice to produce native fatty alcohols by direct hydrogenation of thefree fatty acids.

It is known from Ullman, loc. cit., that the hydrogenation of free fattyacids to fatty alcohols can be carried out by the suspension processusing copper (II) chromite catalysts. However, this method can only beeffectively used when the copper (II) chromite catalyst is obtained bydecomposition of the copper ammonium chromate complex initially obtainedand subsequent washing with acetic acid. Catalysts prepared in this wayare particularly expensive and, in practice, can only be used forsuspension hydrogenation. Acid-washed copper (II) chromite can only betabletted with considerable difficulty, if at all, and accordinglycannot be converted into abrasion-resistant or mechanically strongextrudates or other shapes. Any attempt to achieve this increase instrength by after-annealing weakens the effect of the catalyst. Attemptsto apply acid-washed copper (II) chromite to catalyst supports, such assilica gel or aluminium oxide for example also produces technicallyunuseable catalysts. The support is attacked and the catalyst is readilywashed away.

According to the relevant patent literature, fatty acid esters, moreespecially fatty acid methyl ester, and free fatty acids are thereforesimultaneously used as starting materials for the hydrogenation reactionto saturated and/or unsaturated fatty alcohols (cf. for example U.S.Pat. No. 3,193,586; U.S. Pat. No. 3,173,959; German Pat. Nos. 2,513,377and 2,613,226). So far as industrial application is concerned the aboveprocesses have to be evaluated entirely differently according to whetherthe fatty acid esters of the free fatty acids are used as startingmaterial for hydrogenation. It is generally known that the significantadvantages of fixed-bed catalysis using solid catalysts do not apply tothe processing of a starting material consisting of or containing freefatty acids. The corrosive effect of the free fatty acids at hightemperatures and pressures on the solid catalysts which, basically, havebeen successfully used in the reduction of methyl esters is so greatthat it has not hitherto been possible to consider any of the aboveproposals for practical application in the reduction of free acids.

In practice, therefore, the situation is remedied by initiallyintroducing relatively large quantities of fatty alcohol into thehydrogenation reactor and subsequently adding free fatty acid underhydrogenation conditions. However, this process requires relativelylarge reactors. It only achieves conversions of 96% whereas processesusing fixed-bed catalysts achieve conversions of 99% and higher.

As discussed above, copper chromite catalysts are highly activecatalysts for the hydrogenation of fatty acid esters and triglyceridesof oils and fats. The direct hydrogenation of fatty acids with catalystsof this type has not hitherto been possible because copper (II) chromite(CuCr₂ O₄) is not acid-resistant, i.e. is dissolved by the action of thefree fatty acid. This applies in particular to the CuO present in thecatalyst.

DESCRIPTION OF THE INVENTION

Other than in the operating examples, or where otherwise indicated, allnumbers expressing quantities of ingredients or reaction conditions usedherein are to be understood as modified in all instances by the term"about".

Accordingly, an object of the present invention is to convert copper(II) chromite at high temperatures into an acid-resistant spinel formwhile maintaining as large a specific catalyst surface as possible andthe corresponding pore structure which guarantees high activity and ahigh volume-time yield.

Accordingly, the present invention relates to a process for theproduction of an acid-resistant copper chromite spinel catalyst for thefixed-bed hydrogenation of fatty acids to fatty alcohols ofcorresponding chain length, wherein

(a) silica containing copper (II) chromite (CuCr₂ O₄) is prepared bydissolving a water soluble copper salt, e.g. the nitrate, preferably thenitrate salt, in water, optionally in the presence of other watersoluble metal salts such as barium and manganese, also preferably in theform of their nitrate salts, adding thereto an aqueous colloidalsolution of SiO₂ in a quantity such that from 0.1 to 15%, preferably 0.5to 15%, and more preferably 4.5%, by weight of SiO₂ is present, based onthe weight of the catalyst as a whole. The above salt-SiO₂ solution isthen mixed with an aqueous chromic acid/ammonia solution in amountsufficient to convert all of the copper present to copper chromate,which precipitates out of the solution and contains the SiO₂ intimatelyadmixed therewith;

(b) the copper chromate is washed free of soluble anions (e.g. nitrates)with water, dried, and calcined for at least 12 hours (e.g. from 12 to24 hours) at a temperature of at least 750° C. (e.g. from 750° to 1000°C.).

In one embodiment of the present invention, the resulting calcinedpowder is mixed with from 0.5 to 10% by weight of an organic binder,such as for example polyvinyl acetate or methyl methacrylate, and from0.5 to 15% by weight graphite, and the resulting mixture is granulatedand subsequently tabletted; the percentages by weight being based on theweight of the mixture.

The present invention also relates to the acid-resistant copper (II)chromite spinel catalyst which has a silica gel content of from 0.1 to15% by weight, based on the weight of the catalyst as a whole.

In one particular embodiment of the copper chromite spinel catalystaccording to the invention, the silica gel content is 4.5% by weight.

The catalyst of the invention is suitable for the direct fixed-bedhydrogenation of fatty acids to fatty alcohols of corresponding chainlength.

In one embodiment of the invention, C₆ -C₂₄ fatty acid mixtures whichcan be obtained from animal and/or vegetable fats and/or oils are usedas starting materials. The direct hydrogenation process according to theinvention can be carried out using a ratio by volume of the quantity ofcatalyst to fatty acid mixture of from 0.1 to 3 per hour.

According to the invention, therefore, it is possible to use the knownacid stability of copper (I) spinel for the direct hydrogenation offatty acids to fatty alcohols on an industrial scale without, at thesame time, having to accept losses in the volumetime yield. The porevolume time yields indicated in the literature for copper (I) spinelshave been unexpectedly improved by the process according to theinvention in which the porosity of the spinel is maintained.

In tests conducted with the new catalysts, it was found that an optimumin regard to activity and acid resistance is obtained when the SiO₂content is 4.5% by weight.

In contrast to the copper (II) chromite catalyst, the copper (II) spinelcatalyst according to the invention can readily be tabletted and,according, can be used as a fixed-bed catalyst.

Surprisingly, the process of the invention for the direct catalytichydrogenation of fatty acids can also be improved by increasing theeffectiveness of the catalyst by comparison with commercial catalyststhrough optimization not only of the chemical composition of thecatalyst, as described above, but also of the physical structure of theshaped elements of the catalyst. By this is meant that, surprisingly,the physical structure of the shaped catalyst elements also has aconsiderable bearing on the activity and selectivity of the catalyst.According to the invention, an improvement in the process was obtainedby tabletting the catalyst using 1 to 10% by weight of one or morebinders. Compounds known from the prior art may be used for thispurpose. One or even several binders may be present in the catalyst ofthe invention. It has proven to be particularly effective to use one ormore binders from the group comprising polyvinyl acetate and methylmethacrylate. Polyvinyl acetate is preferably used as binder for theproduction of catalyst tablets; for example, commercially available 10%by weight polyvinyl acetate suspensions may be used for the productionof the catalyst. The polyvinyl acetate suspensions are added in smallquantities to the calcined, powder-form catalyst materials and mixedtherewith until agglomerate grains begin to build up. Thereafter, theagglomerate-containing powder is compacted into small granulates, forexample in a perforated roll granulator. The granulates are dried inknown manner to residual moisture contents of from 10 to 15%. Theresulting granulates are sieved and tabletted.

Before they are used in the direct hydrogenation of fatty acids, thecatalyst used in the process of the invention are activated withhydrogen or with a hydrogen-containing gas mixture. A gas mixturepredominantly consisting of a nitrogen/hydrogen gas mixture isadvantageously used to activate the catalyst masses. As known from theprior art, such activation may advantageously be carried out by dryingthe catalyst masses in a stream of nitrogen at elevated temperatureafter their production and adding hydrogen in increasing quantities tothe drying gas for activation. The hydrogen content of the activatinggas mixture may be from 0.1 to 10% by volume. The catalysts may beactivated both in situ and in vessels separate from the reaction vessel.

It is known that, depending on their origin, native fats and oilscontained from relatively small to relatively large amounts of mono- orpolyolefinically unsaturated fatty acids. Since the copper alsosaturates double bonds, saturated alcohols are also formed fromunsaturated fatty acid esters.

By monitoring the quality of the end product of the process, the correcttemperature can readily be selected in coordination with the ratio ofrecycled hydrogen to starting material. Excessively low processtemperatures lead to a corrosive attack on the fixed-bed catalyst andhence to the discharge of metal soap via the reaction product. Bothchromium soaps and also copper soaps may accumulate. Excessively highprocess temperatures lead to over-reduction and hence to an undesirablyhigh formation of paraffins. Process temperatures are usually in therange of from 200° C. to 400° C., preferably in the range of from 250°C. to 300° C.

The reaction parameters to be adjusted in each individual case areco-determined inter alia by the length of the carbon chains of the fattyacids or fatty acid mixtures to be reduced. The shorter the chain lengthof the fatty acids used, the lower generally are the reactiontemperatures within the ranges indicated.

In principle, lower alcohols, low-boiling paraffins or steam may beadded to fatty acid starting materials used in accordance with theinvention to modify the process conditions. However, provision must bemade to ensure that the gaseous secondary reaction products formed arealso removed from the reaction circuit and, in particular, from therecycled gas. The secondary reaction products are, in particular, thewater formed during the reaction, small quantities of hydrocarbons andthe quantities of nitrogen inevitably brought in with the hydrogensubsequently introduced.

The process is generally carried out under a pressure of 200 bar orhigher and more especially under a pressure in the range of from 200 to500 bar. In principle, the use of relatively high pressures leads to areduction in the acid value of the reaction products and hence to anincrease in the yield of the desired fatty alcohols.

The following examples illustrate but are not meant to limit theinvention.

Example 1 below describes the production of a particularly suitablecopper (I) spinel which is used in Examples 2 and 3.

EXAMPLE 1

85 g barium nitrate, 294 g manganese nitrate and 2493 g copper nitratewere dissolved in 9 liters deionized water. 550 g of a 40% SiO₂ colloidwere added to the clear solution. The mixture was heated to 70° C.

In a second stirring vessel, 1639 g chromic acid were dissolved in 9liters water. 3600 g of a 25% ammonium solution were added to theresulting solution. The solution was heated to 70° C. The copperchromate was precipitated by addition of the Cu, Ba, Mn nitrate solutionto the ammonium chromate solution introduced beforehand. The filter cakewas washed free from nitrate and dried. Calcination was carried out for12 hours at 750° C. The powder was then mixed with 2% polyvinyl acetateand 2% graphite, granulated and tabletted (4×4 mm tablets).

EXAMPLE 2

12 g powder of the catalyst prepared in accordance with Example 1 and600 g C₁₂ fatty acid were reacted in a 2 liter autoclave at atemperature of 260° C. under a hydrogen pressure of 250 bar. Thefollowing fatty acid conversions were determined after 1, 3 and 5 hours.

    ______________________________________                                        Reaction time (h):                                                                           1           3     5                                            FA conversion (%):                                                                           39          48    75                                           ______________________________________                                    

The catalyst was not dissolved during the hydrogenation reaction.

EXAMPLE 3

In a tube reactor, tablets (4×4 mm) in volume of 500 ml were reducedwith an H₂ /N₂ mixture (H₂ : N₂ =1:10) at 200° C. The catalyst wascarefully warmed up with C₁₂ -C₁₈ fatty acid methyl ester. 400 to 500 1C₁₂ fatty acid per hour were then pumped over the catalyst under ahydrogen pressure of 250 bar and at a temperature of 260° to 275° C. Theeffluents were water-clear. The following values were determined:

    ______________________________________                                        Acid value:            0.04                                                   Saponification value:  approx. 2                                              Hydroxyl value:        285 to 293                                             ______________________________________                                    

We claim:
 1. A process for the production of an acid-resistant copperchromite spinel catalyst consisting essentially of copper (II) chromiteand silica wherein copper (II) is the only metal ion present thereincomprising the steps of:(a) dissolving a water soluble copper salt inwater to form an aqueous solution thereof, (b) mixing the above aqueoussolution with an aqueous colloidal solution of SiO₂, (c) mixing thesolution resulting from step (b) with an aqueous chromic acid/ammoniasolution in amount sufficient to form a precipitate of substantially allof the copper present as copper chromate, (d) washing the precipitatedchromate with water to remove soluble anions therefrom, (e) drying thecopper chromate, and (f) calcining the copper chromate for at leastabout 12 hours at a temperature of at least about 750° C. to producesilica containing copper (II) chromite;wherein in step (b) the SiO₂ ispresent in a quantity of from about 0.1 to about 15% by weight, based onthe weight of the catalyst as a whole.
 2. The process of claim 1 whereinin step (a) the water soluble copper salt is the nitrate salt.
 3. Theprocess of claim 1 wherein from about 0.5 to about 15% by weight of SiO₂is present in step (b).
 4. The process of claim 1 wherein about 4.5%SiO₂ is present in step (b).
 5. The process of claim 1 wherein thesilica containing copper (II) chromite from step (f) is then mixed withfrom about 0.5 to about 10% by weight of an organic binder, and fromabout 0.5 to about 15% by weight of graphite, based on the weight of themixture, and the mixture is then granulated and formed into tablets. 6.The process of claim 5 wherein the organic binder is polyvinyl acetateor methyl methacrylate.
 7. An acid-resistant copper (II) chromite spinelcatalyst containing intimately admixed therewith from about 0.1 to about15% by weight of SiO₂, based on the weight of the catalyst as a whole.8. The catalyst of claim 7 wherein from about 0.5 to about 15% SiO₂ ispresent therein.
 9. The catalyst of claim 7 wherein about 4.5% SiO₂ ispresent therein.
 10. An acid-resistant copper (II) chromite spinelcatalyst in tablet form comprisingA. copper (II) chromite containingintimately mixed therewith from about 0.1 to about 15% by weight SiO₂,based on the total weight of catalyst, B. from about 0.5 to about 10% byweight, based on the total weight of catalyst, of an organic binder, andC. from about 0.5 to about 15% by weight, based on the total weight ofcatalyst, of graphite.
 11. The catalyst of claim 10 wherein the organicbinder is polyvinyl acetate or methyl methacrylate.
 12. The catalyst ofclaim 10 wherein in component A. from about 0.5 to about 15% by weightof SiO₂ is present.
 13. The catalyst of claim 10 wherein in component A.about 4.5% SiO₂ is present.